External charging device for charging an implantable medical device and methods of regulating duty of cycle of an external charging device

ABSTRACT

In one embodiment, an external charging device for recharging an implanted medical device, comprises: a battery for powering the external charging device; a coil for radiating RF power; drive circuitry for driving the coil according to a duty cycle; circuitry for generating a signal that is indicative of an amount of current flowing through the coil; and control circuitry for controlling the drive circuitry, wherein the control circuitry is operable to process the signal from the circuitry for generating to detect when a coil of the implantable medical device temporarily ceases absorbing RF power, the control circuitry modifying the duty cycle in response to detection of the coil of the implantable medical device temporarily ceasing absorbing RF power.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/035,131, filed Mar. 10, 2008, which is incorporated herein byreference.

TECHNICAL FIELD

The present application is generally related to systems and methods forregulating a duty cycle for an external charging device that is chargingan implantable medical device.

BACKGROUND

Neurostimulation systems are devices that generate electrical pulses anddeliver the pulses to nerve tissue to treat a variety of disorders.Spinal cord stimulation (SCS) is an example of neurostimulation in whichelectrical pulses are delivered to nerve tissue in the spine for thepurpose of chronic pain control. Other examples include deep brainstimulation, cortical stimulation, cochlear nerve stimulation,peripheral nerve stimulation, vagal nerve stimulation, sacral nervestimulation, etc. While a precise understanding of the interactionbetween the applied electrical energy and the nervous tissue is notfully appreciated, it is known that application of an electrical fieldto spinal nervous tissue can effectively mask certain types of paintransmitted from regions of the body associated with the stimulatednerve tissue. Specifically, applying electrical energy to the spinalcord associated with regions of the body afflicted with chronic pain caninduce “paresthesia” (a subjective sensation of numbness or tingling) inthe afflicted bodily regions. Thereby, paresthesia can effectively maskthe transmission of non-acute pain sensations to the brain.

Neurostimulation systems generally include a pulse generator and one ormore leads. The pulse generator is typically implemented using ametallic housing that encloses circuitry for generating the electricalpulses, control circuitry, communication circuitry, a rechargeablebattery, recharging circuitry, etc. The pulse generation circuitry iscoupled to one or more stimulation leads through electrical connectionsprovided in a “header” of the pulse generator. Stimulation leadstypically include multiple wire conductors enclosed or embedded within alead body of insulative material. Terminals and electrodes are locatedon the proximal and distal ends of the leads. The conductors of theleads electrically couple the terminals to the electrodes. Theelectrical pulses from the pulse generator are conducted through theleads and applied to patient tissue by the electrodes of the leads.

Recharging of an implanted pulse generator typically occurs bynear-field coupling of a coil in the implanted pulse generator with acoil of an external charging device (which could also function as aprogramming device). The external charging device radiates power fromits coil which induces current in the coil of the implanted pulsegenerator. The recharging circuitry of the implanted pulse generatorrectifies the induced current and charges the battery of the implantedpulse generator (subject to various regulation circuitry).

SUMMARY

In one embodiment, an external charging device for recharging animplanted medical device, comprises: a battery for powering the externalcharging device; a coil for radiating RF power; drive circuitry fordriving the coil according to a duty cycle; circuitry for generating asignal that is indicative of an amount of current flowing through thecoil; and control circuitry for controlling the drive circuitry, whereinthe control circuitry is operable to process the signal from thecircuitry for generating to detect when a coil of the implantablemedical device temporarily ceases absorbing RF power, the controlcircuitry modifying the duty cycle in response to detection of the coilof the implantable medical device temporarily ceasing absorbing RFpower.

The foregoing has outlined rather broadly certain features and/ortechnical advantages in order that the detailed description that followsmay be better understood. Additional features and/or advantages will bedescribed hereinafter which form the subject of the claims. It should beappreciated by those skilled in the art that the conception and specificembodiment disclosed may be readily utilized as a basis for modifying ordesigning other structures for carrying out the same purposes. It shouldalso be realized by those skilled in the art that such equivalentconstructions do not depart from the spirit and scope of the appendedclaims. The novel features, both as to organization and method ofoperation, together with further objects and advantages will be betterunderstood from the following description when considered in connectionwith the accompanying figures. It is to be expressly understood,however, that each of the figures is provided for the purpose ofillustration and description only and is not intended as a definition ofthe limits of the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts block diagrams of an external charging device and animplantable pulse generator according to one representative embodiment.

FIG. 2 depicts a block diagram of charge control and communicationcircuitry of an implantable pulse generator according to onerepresentative embodiment.

FIG. 3 depicts circuitry for controlling a charging coil within animplantable pulse generator according to one representative embodiment.

FIG. 4 depicts a charging wand coupled to selected circuitry of anexternal charger according to one representative embodiment.

FIG. 5 depicts a duty cycle for driving a coil of an external chargingdevice according to one representative embodiment.

FIG. 6 depicts a signal indicative of current flowing through a coil ofa charging wand according to one representative embodiment.

FIG. 7 depicts a flowchart for conducting charging operations accordingto one representative embodiment.

DETAILED DESCRIPTION

FIG. 1 depicts block diagrams of external charger 100 and implantablepulse generator (IPG) 150 according to one representative embodiment.Charger 100 comprises controller 101 (e.g., any suitable commerciallyavailable microcontroller) for controlling the operations of charger 100according to instructions stored in non-volatile memory 102. Charger 100is powered by battery 104. In preferred embodiments, battery 104 is arechargeable lithium ion battery. External charger 100 further comprisescharging and communication circuitry 103. Charging and communicationcircuitry 103 may be adapted, in some embodiments, to electricallycouple to a coil of an external wand (shown in FIG. 4) that is held,during charging, by the patient about the patient's body immediatelyadjacent to the implant site of IPG 150. Alternatively, the coil may beintegrated in the same device package with the circuitry of charger 100.Charging and communication circuitry 103 drives the coil using asuitable RF signal for charging purposes. Charging and communicationcircuitry 103 also drives the coil using a suitable modulated RF signalto communicate data to IPG 150. Charger 100 may also be adapted for useas a controller to control the operations of IPG 150 by communicatingsuitable control parameters using circuitry 103.

IPG 150 comprises controller 151 (e.g., any suitable commerciallyavailable microcontroller) for controlling the pulse generating andother operations of IPG 150 according to instructions stored innon-volatile memory 152. IPG 150 comprises pulse generating circuitry153 for generating stimulation pulses for delivery to tissue of thepatient. Any suitable existing or later developed pulse generatingcircuitry may be employed. An example of pulse generating circuitry isdescribed in U.S. Patent Application Publication No. 2006/0259098,entitled “SYSTEMS AND METHODS FOR USE IN PULSE GENERATION,” which isincorporated herein by reference. Pulse generating circuitry 153 maycomprise one or multiple pulse sources. Also, pulse generating circuitry153 may operate according to constant voltage stimulation, constantcurrent stimulation, or any other suitable mode of operation. IPG 150may be adapted for spinal cord stimulation, peripheral nervestimulation, peripheral nerve field stimulation, deep brain stimulation,cortical stimulation, gastric pacing, cardiac therapies, and/or thelike. Although an IPG is discussed in regard to one embodiment,representative embodiments may be employed to recharge any type ofimplantable medical device.

The various components of IPG 150 are powered by battery 154 (preferablya lithium ion rechargeable battery). Battery 154 is recharged byconverting RF power radiated from external charger 100. Charging andcommunication circuitry 155 preferably comprises a coil (shown in FIG.3) for near-field coupling with the coil of external charger 100. Whenexternal charger 100 radiates RF power using its coil, the inductivecoupling between the coil of charger 100 with the coil of IPG 150 causescurrent to be induced in the coil of IPG 150. Circuitry 155 uses theinduced current to charge battery 154. Also, circuitry 155 preferablyuses the same coil to communicate with charger 100.

FIG. 2 depicts a block diagram of circuitry 155 according to onerepresentative embodiment. Circuitry 155 comprises coil and bridgerectifier circuitry 203. The coil of circuitry 203 is used both forcharging operations and for communication with charger 100. Near fieldreceiver 201 is coupled to the coil (through capacitor 302 shown in FIG.3) to the coil. Receiver 201 demodulates data when a carrier at anappropriate frequency is detected. Receiver 201 communicates a serialdata stream to controller 151. Near field transmitter 202 receives aserial data stream from controller 151 and generates a modulated RFsignal for application to coil 301 to communicate data to charger 100.Signal modulation and demodulation may, alternatively, be implemented insoftware executing on controller 151. In preferred embodiments, nearfield receiver 201 and transmitter 202 do not operate when chargingoperations are taking place. Accordingly, charger transmitter 205 isemployed to provide charging status messages to charger 100 whencharging is occurring.

Bridge measure circuitry 204 measures the output voltage (the voltage onVCHG 309 in FIG. 3) of circuitry 155 for control of charging operations.Primary regulatory circuitry 207 preferably operates to control chargingoperations in response to the measurement signal from circuitry 204.When the output voltage is relatively low, regulatory circuitry 207permits circuitry 155 to absorb RF power. When the output voltage isrelatively high, the coil is shorted to ground to prevent absorption ofRF power.

Charge control circuitry 206 controls the charging of battery 154.Charge control circuitry 206 uses the measurement functionality ofbattery measurement circuitry 208 to detect the state of battery 154.Battery measurement circuitry 208 may measure the battery voltage,charging current, battery discharge current, and/or the like. Using thebattery voltage measurement of circuitry 208, charge control circuitry206 may prevent battery charging when an end-of-life (EOL) state hasbeen reached for battery 154. Also, charge control circuitry 206 may usea number of measurements to conduct fast charging operations asdisclosed in greater detail in U.S. Patent Application Publication No.2006/0259098, entitled “SYSTEMS AND METHODS FOR USE IN PULSEGENERATION.” Charge control circuitry 206 also preferably monitors theoutput signal from bridge measure circuitry 204 to further regulate theoutput voltage from coil and bridge rectifier circuitry 203.

FIG. 3 depicts a portion of charging and communication circuitry 155 inmore detail according to one representative embodiment. As shown in FIG.3, circuitry 155 comprises coil 301 for inductively coupling with thecoil of charger 100. Specifically, coil 301 and capacitors 302 and 303are preferably tuned to capture RF power at one or more frequencies. Inone preferred embodiment, coil 301 and capacitors 302 and 303 are tunedto receive power at a first RF frequency from external charger 100 andat a second RF frequency from a separate physician-patient programmerdevice (not shown).

The RF power is rectified by bridge rectifier 304. The output ofrectifier 304 is shown in FIG. 3 as the node VCHG 309. The voltage onVCHG 309 is used to charge the battery assuming all necessary conditionsare met. FETs 307 and 308 are used by primary regulatory circuitry 207to regulate the voltage on VCHG 309 during charging operations. In oneembodiment, primary regulatory circuitry 207 employs a band-gapcomparison to regulate the voltage on VCHG 309. When the voltage isbelow the bottom threshold value (e.g., 4.77V) of the band-gap,regulatory circuitry 207 turns off FETs 307 and 308 and coil 301 absorbsRF power. When the voltage is above the top threshold value (e.g.,4.93V) of the band-gap, regulatory circuitry 207 turns on FETs 307 and308 to short coil 301 to ground thereby preventing absorption of RFpower.

Charge control circuitry 206 uses FETs 305 and 306 to respond to anerror condition or to prevent an over-voltage condition on VCHG 309. Inone embodiment, when the voltage on VCHG 309 is above approximately6.5V, charge control circuitry 206 clamps the bridge inputs using FETs305 and 306 to ground to stop energy absorption by coil 301 as aredundant safety mechanism.

During charging operations, status messages are communicated by chargertransmitter 205 using FETs 305 and 306. The one-way communication occursby controlling a 3 kHz modulation of coil 301 by charger transmitter205. When communication of a status message is desired, chargertransmitter 205 toggles its output to FETs 305 and 306 at 3 kHz with adata rate set at 300 baud. Error conditions and a charge-completecondition are examples of charging states that can be communicated usingcharger transmitter 205.

FIG. 4 depicts charging wand 401 coupled to selected circuitry ofexternal charger 100 according to one representative embodiment.Charging wand 401 comprises coil 402 and capacitors 403 and 404. Coil402 and capacitors 403 and 404 are preferably tuned to radiate RF powerefficiently at the frequency selected for charging IPG 150. Drivecircuitry 405 generates an RF signal at the selected frequency to drivecoil 402. Current detection circuitry 406 is used to detect the currentflowing through coil 402. Current detection circuitry 406 can beimplemented in any number of ways. For example, a current transformermay be employed to detect the current flowing through coil 402 accordingto one embodiment. Alternatively, a connection to ground through one ormore high resistance resistors could be used to facilitate the currentmeasurement.

In representative embodiments, drive circuitry 405, under the control ofcontroller 151, drives coil 402 at the selected frequency according to aduty cycle. As shown in FIG. 5, drive signal 500 is preferably appliedto drive circuitry 405. In FIG. 5, drive signal 500 is low at times 501,503, and 505. When drive signal 500 is low, drive circuitry 405 does notgenerate the RF signal to drive coil 402. Drive signal 500 is high attimes 502, 504, and 506. When drive signal 500 is high, drive circuitry405 generates the RF signal to drive coil 402. The duty cycle definesthe amount of time that drive signal 500 is high relative to the amountof time that drive signal 500 is low.

The duty cycle of drive signal 500 is preferably controlled bycontroller 151 to expend a minimal amount of power charging IPG 150. Aspreviously noted, during charging operations, IPG 150 operates to clampits coil 301 to ground when a charging voltage exceeds a certain limit.When coil 301 of IPG 150 is clamped to ground, the RF power radiated byexternal charger 100 is wasted, because no usable power is coupled intocoil 301 of IPG 150. By modifying the duty cycle of the RF power signalradiated by external charger 100, the amount of time that coil 301 ofIPG 150 is clamped to ground can be minimized, thereby minimizing thepower wasted by external charger 100.

When charging wand 401 is placed within relatively close proximity ofIPG 150, the current flowing through coil 402 depends upon whether coil301 is clamped to ground or not. FIG. 6 depicts signal 600 generated bycurrent detection circuitry 406 according to one representativeembodiment. As shown in FIG. 6, time segments 601, 603, and 605 indicatean amount of current flowing through coil 402 while coil 301 of IPG 150is absorbing power (i.e., is not clamped to ground). Time segments 602and 604 indicate an amount of current flowing through coil 402 whilecoil 301 of IPG 150 is not absorbing power (i.e., is clamped to ground).Coupling interval 606 represents an amount of time between respectiveoccasions when coil 301 is not absorbing power. The current flowingthrough coil 402 may increase or decrease when coil 301 is clamped toground. The change in current will depend upon the tuning of coil 402and capacitors 403 and 404 relative to the selected frequency for the RFpower signal.

If coupling interval 606 is relatively short, external charger 100 isradiating excessive power on average. The power radiated by externalcharger 100 is repeatedly causing the voltage on VCHG 309 to exceed adefined amount. In that case, modifying the duty cycle of drivecircuitry 405 permits charging to occur in a more efficient manner fromthe perspective of external charger 100. That is, the duty cycle can bemodified to cause drive circuitry 405 to spend less time generating theRF power signal and, hence, to expend less energy in charging IPG 150.

FIG. 7 depicts a flowchart for operating an external charging device forcharging an implantable medical device according to one representativeembodiment. In some embodiments, appropriate portions of the flowchartof FIG. 7 can be implemented using software instructions or codeexecuting on a controller or processor of the external charging deviceto control the various hardware circuits of the external chargingdevice.

In 701, a user positions a charging wand adjacent to the patient's bodyat a location near to the implant site of the implantable medical deviceto be recharged. In 702, the user selects an option on the externalcharging device to begin charging operations.

In 703, an initial duty cycle is selected or defined for chargingoperations. In 704, charging operations occur by driving the coil of thecharging wand at a suitable RF frequency according to the selected dutycycle.

In 705, a signal indicative of the current flowing through the coil isgenerated. In 706, the signal is provided to a demodulation circuit todetect charging status messages communicated from the implanted medicaldevice. Alternatively, the signal may be sampled and processed by asuitable signal processing routine to demodulate the charging statusmessages. The signal can be sampled using an on-chip analog-to-digitalconverter of the microcontroller of the external charging device.

In 707, a logical comparison is made to determine whether a statusmessage indicates the charging operations should be terminated. If so,charging stops and a suitable message is provided to the user (708). Themessage may indicate to the user that the battery of the implantablemedical device is fully charged, the battery has reached an end-of-life(EOL) state and should be replaced, or some error or malfunction hastaken place. If the logical comparison indicates that charging shouldcontinue, the process flow continues to 709.

In 709, a logical comparison is made to determine whether additionalpower is appropriate. If so, the process flow proceeds to 710 where theduty cycle is modified to drive the coil a greater amount of time. From710, the process flow returns to 704 to continue charging operationsaccording to the newly selected duty cycle. If the logical comparisondetermines that additional power is not appropriate, the process flowproceeds from 709 to 711.

In 711, the signal indicative of current flow through the coil isprocessed to identify whether the coil of the implantable medical devicehas ceased absorbing RF power. If so, the process flow continues to 712.If not, the process flow returns to 704 to continue charging operations.

In 712, an amount of time is calculated since the preceding time thatthe coil of the implantable medical device ceased absorbing RF power. In713, the calculated amount of time is compared to a threshold value. Ifthe amount of time is greater than the threshold value, the process flowreturns to 704 to continue charging operations. If the amount of time isless than the threshold value, the process flow proceeds to 714. In 714,another value is selected for the duty cycle to drive the coil lessoften. The process flow returns to 704 to continue charging operationsaccording to the newly selected duty cycle. In an alternativeembodiment, an averaging algorithm could be employed in which the totalnumber of times that the coil of the implantable medical device ceasedabsorbing RF power within a time window is used to determine whether tomodify the duty cycle.

Although certain representative embodiments and advantages have beendescribed in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the appended claims. Moreover, the scope of thepresent application is not intended to be limited to the particularembodiments of the process, machine, manufacture, composition of matter,means, methods and steps described in the specification. As one ofordinary skill in the art will readily appreciate when reading thepresent application, other processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed that perform substantially the same function orachieve substantially the same result as the described embodiments maybe utilized. Accordingly, the appended claims are intended to includewithin their scope such processes, machines, manufacture, compositionsof matter, means, methods, or steps.

1. A stimulation system, comprising: (i) an implantable pulse generator(IPG) that comprises: a first rechargeable battery for powering the IPG;pulse generating circuitry; a first coil for receiving RF power; arectifier coupled to the coil; and first control circuitry forcontrolling RF power conversion, wherein the first control circuitryprevents the coil from absorbing RF power for conversion by therectifier when a power conversion voltage exceeds a predefined level;and (ii) an external charging device for recharging the IPG, comprising:a second rechargeable battery for powering the external charging device;a second coil for radiating RF power; drive circuitry for driving thesecond coil according to a duty cycle; circuitry for generating a signalthat is indicative of an amount of current flowing through the secondcoil; and second control circuitry for controlling the drive circuitry,wherein the second control circuitry is operable to process the signalfrom the circuitry for generating to detect when the first coiltemporarily ceases absorbing RF power, the second control circuitrymodifying the duty cycle in response to detection of the first coiltemporarily ceasing absorbing RF power.
 2. The system of claim 1 whereinthe second control circuitry comprises a microcontroller for executingcode that processes samples of the signal from the circuitry forgenerating.
 3. The system of claim 1 wherein the second controlcircuitry is operable to measure an amount of time between successiveoccasions when the first coil temporarily ceases absorbing RF power. 4.The system of claim 1 wherein the second control circuitry is operableto count a number of times that the first coil temporarily ceasesabsorbing RF power within a defined amount of time.
 5. The system ofclaim 1 wherein the second rechargeable battery is a rechargeablelithium ion battery.
 6. The system of claim 1 wherein the second coil isdisposed in a removable charging wand.
 7. The system of claim 1 whereinthe first control circuitry is operable to clamp the first coil toground to cause the first coil to temporarily cease absorbing RF power.8. A method of charging a medical device implanted with a human bodyusing an external charging device, comprising: selecting a duty cycle onthe external charging device; driving a first coil, by the externalcharging device, to radiate RF power according to the selected dutycycle; absorbing RF power by a second coil in the implanted medicaldevice; converting the radiated RF power in the implanted medical deviceto electrical power; detecting, in the implanted medical device, that aRF conversion voltage has exceeded a defined limit; temporarily ceasingabsorption of RF power by the second coil for conversion to electricalpower in the implanted medical device; detecting, by the externalcharging device, when the second coil of the implantable medical devicetemporarily ceases absorbing RF power; and modifying the duty cycle fordriving the first coil in response to the detecting when the coil of theimplantable medical device temporarily ceases absorbing RF power.
 9. Themethod of claim 8 wherein the detecting when the second coil of theimplantable medical device temporarily ceases absorbing RF power furthercomprises: measuring an amount of time between successive occasions whenthe first coil temporarily ceases absorbing RF power.
 10. The method ofclaim 8 wherein the detecting when the second coil of the implantablemedical device temporarily ceases absorbing RF power further comprises:counting a number of times that the first coil temporarily ceasesabsorbing RF power within a defined amount of time.
 11. The method ofclaim 8 wherein the external charging device is powered by arechargeable lithium ion battery.
 12. The method of claim 8 wherein thefirst coil is disposed in a removable charging wand.
 13. The method ofclaim 8 wherein the temporarily ceasing absorption of RF powercomprises: clamping the first coil to ground to cause the first coil totemporarily cease absorbing RF power.
 14. The method of claim 8 whereinthe duty cycle is selected by modified by a microcontroller of theexternal charging device.